This invention relates in general to a threaded connection between two tubular elements, each having a pin member on one end and a box member on the other end. In particular, this invention relates to a threaded connection for use with offshore riser pipe.
In offshore production applications, a plurality of tubular riser elements are joined together in an end-to-end configuration and extend from a subsea well assembly to a surface platform. Advances in drilling technology have made it possible to drill at greater water depths, subjecting the production risers to extremely high pressures and bending loads. Metal-to-metal seals can provide an effective seal under these harsh conditions; however, contamination, pitting, or damage to the seal surfaces causes these seals to deteriorate rapidly. Thermoplastic seal elements can be used as secondary sealing elements; however, these components typically resist radial compression and therefore create internal forces which act to urge the box and pin elements apart. Bending loads due to currents and wave motion can cause the metal seals to cycle, creating fatigue. This in turn reduces the effectiveness of the metal-to-metal seals, contributing to the failure of the connection.
Attempts have been made to overcome these problems. For example, U.S. Pat. No. 4,707,001 discloses a connection featuring multi-start threads with a reverse angle load flank in conjunction with a torque shoulder seal to lock the pin and box against radial separation. While this design may be workable, the torque shoulder could create plastic deformation in the box or pin member if too much torque is applied to the connection. This design is also susceptible to stress fractures because the torque shoulder seal applies repetitive bending loads to the box and pin.
The connection of this invention features a box member with an internal thread an a pin member with a cooperative external thread. A first metal-to-metal seal located adjacent to the box shoulder forms the primary internal seal of the connection. A second metal-to-metal seal located near the box end forms the primary external seal of the connection. Guide surfaces located on the pin end and box end protect the metal sealing surfaces from damage during makeup.
A dual angle torque shoulder in combination with a large blend radius is located on the pin member for engaging a corresponding dual angle box end, locking the box end securely into place against the pin. This configuration is self-centering. The dual angle torque shoulder has inner and outer inclined surfaces that join each other with a large blend radius. The dual angle torque shoulder in combination with the large blend radius self-centers, prevents radial distention of the box end, and maintains bearing pressure between the external sealing surfaces. Because the torque shoulder has a dual angle configuration, the pin and box will not undergo plastic deformation as a result of excessive torque. The torque shoulder on the box end is generally torroidal, being convex in cross-section.
The connection also has stress relief grooves located on the box member and on the pin member. These stress relief grooves reduce the incidence of stress fractures, thereby improving the fatigue life of the connection.